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• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/042—Electrodes formed of a single material
  • C25B11/043—Carbon, e.g. diamond or graphene
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
  • B01J27/02—Sulfur, selenium or tellurium; Compounds thereof
  • B01J27/04—Sulfides
  • B01J27/043—Sulfides with iron group metals or platinum group metals
  • B01J27/045—Platinum group metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/24—Halogens or compounds thereof
  • C25B1/26—Chlorine; Compounds thereof
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/091—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds
  • C25B11/093—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds at least one noble metal or noble metal oxide and at least one non-noble metal oxide
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/8605—Porous electrodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
  • H01M4/923—Compounds thereof with non-metallic elements
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
  • H01M4/925—Metals of platinum group supported on carriers, e.g. powder carriers
  • H01M4/926—Metals of platinum group supported on carriers, e.g. powder carriers on carbon or graphite
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/10—Process efficiency
  • Y02P20/129—Energy recovery, e.g. by cogeneration, H2recovery or pressure recovery turbines
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/20—Improvements relating to chlorine production

Definitions

• the invention relates to a catalyst, in particular to an electrocatalyst for oxygen reduction suitable for incorporation in a gas-diffusion electrode structure, and to a method for manufacturing the same.
• Noble metal sulphide electrocatalysts of the prior art are for instance prepared by sparging hydrogen sulphide in an aqueous solution of a corresponding noble metal precursor, usually a chloride, for instance as disclosed in US 6,149,782, entirely incorporated herein as reference, which is relative to a rhodium sulphide catalyst.
• a corresponding noble metal precursor usually a chloride
• the synthesis of noble metal sulphide catalysts with hydrogen sulphide in aqueous solutions is conveniently carried out in the presence of a conductive carrier, in most of the cases consisting of carbon particles.
• the noble metal sulphide is selectively precipitated on the carbon particle surface, and the resulting product is a carbon-supported catalyst, which is particularly suitable for being incorporated in gas- diffusion electrode structures characterised by high efficiency at reduced noble metal loadings.
• High surface carbon blacks such as Vulcan XC-72 from Cabot Corp./USA are particularly fit to the scope.
• a different procedure for the preparation of carbon-supported noble metal sulphide catalysts consists of an incipient wetness impregnation of the carbon carrier with a solution of a noble metal precursor salt, for instance a noble metal chloride, followed
• M/49021-PCT by solvent evaporation and gas-phase reaction under diluted hydrogen sulphide at ambient or higher temperature, whereby the sulphide is formed in a stable phase.
• This is for instance disclosed in US 2004/0242412, relating to a ruthenium sulphide catalyst.
• a more advanced manufacturing process for noble metal sulphide catalysts is further disclosed in US 6,967,185, entirely incorporated herein as reference, and consists of reacting a noble metal precursor with a thio-compound in an aqueous solution free of sulphide ions; in this way, a catalyst substantially equivalent to the one of US 6,149,782 is obtained avoiding the use of a highly hazardous and noxious reactant such as hydrogen sulphide.
• noble metal sulphides precipitated by the methods of the prior art are all prepared by discrete reduction stages producing a mixture of different crystalline phases with different valences and stoichiometry, some of which present a poor electrochemical activity or none at all.
• some of the most active formulation consist of ternary compounds which cannot be reliably prepared by the environmentally friend method of US 6,967,185; the only viable process for obtaining ternary compounds, such as Ru x Co z S y which is also very attractive in terms of cost, is the one disclosed in US 2004/0242412, still relying on hydrogen sulphide as reactant species.
• M/49021-PCT sulphides are much more stable than any other electrocatalyst for oxygen reduction of the prior art in the hydrochloric acid electrolysis environment, some leakage of noble metal is still detectable, especially when the cell is shut-down for maintenance.
• the invention consists of a catalyst for electrochemical reduction of oxygen comprising a noble metal sulphide supported as a single well-defined crystalline phase on a conductive carbon; preferably, the noble metal catalyst of the invention is a single crystalline phase of a binary or ternary rhodium or ruthenium sulphide.
• Rh 3 S 4 with some amount of metallic rhodium (Rh 0 ).
• RhH 7 Si 5 characterised by a crystal lattice corresponding to the (Pm-3m) space group is the most active, followed by monoclinic (C2/m) Rh 2 S 3 , while the remaining species present little or no activity and in some cases a lesser stability.
• Rh 0 is unstable in hydrochloric acid electrolysis conditions, and accounts for the quickest rhodium leaks during operation.
• the typical amount of RhH 7 Si 5 is a little higher than 70% of the overall rhodium sulphide species.
• a single crystalline phase of (Pm-3m) RhH 7 Si 5 on active carbon can be prepared by suitably modifying the environmentally- friendly manufacturing process disclosed in US 6,967,185.
• the term single crystalline phase is used hereafter to mean a more than 90% pure crystal phase; in the cases of the (Pm-3m) RhH 7 Si 5 catalyst according to the invention, the single crystal phase obtained is about 95% pure with no detectable Rh 0 .
• the method for manufacturing a single crystalline phase of (Pm-3m) RhH 7 Si 5 on active carbon comprises the steps of:
• tetrathionates such as Na 2 S 4 Oe ⁇ 2 H 2 O
• other similar thionate species such as dithionates, trithionates, pentathionates and heptathionates are all fit for this purpose, and also gaseous SO 2 possesses both the reducing power and the sulphur availability to produce amorphous M x S y moieties on a selected support.
• the support carbon particles have preferably a surface area comprised between 200 and 300 m 2 /g, and the preferred specific loading of the resulting rhodium sulphide on carbon is comprised between 12 and 18%.
• the sequence of addition of the reactants is critical to obtain the desired product: to the solution containing the suspended carbon particles and the rhodium precursor salt, the selected sulphur source (for instance a tiosulphate or thionate species) is added, so that the metathesis process can initiate. Simultaneously or immediately after, depending on the specific reaction, a strong reducing agent, defined as a species with a reduction potential below 0.14 V/SHE, is added in small aliquots.
• reducing agent sodium borohydhde (NaBH 4 ) is preferred, but other suitable reactants include LiAIH 4 , hydrazines, formaldehyde and metallic aluminium, zinc or antimony.
• the reducing agent as defined has a reduction potential below the one of S 0 / S "2 couple: in this way, it can achieve the instantaneous metathesis of the metal ions and of the thiosulphate part, directly forming amorphous rhodium sulphide on the carbon support particles while preventing the formation of discrete reduction states, which are the main factor controlling the yield and phase distribution of the different sulphide moieties.
• the method of the invention can be applied to the manufacturing of other single crystalline phases of noble metal sulphides, including not only sulphides of a single metal (binary sulphides) but also of two or more metals (ternary sulphides and so on). This proves particularly useful in the case of ruthenium sulphides, because also in this case the method of the invention gives rise to the most active and stable single crystalline phase.
• binary (RUS2) and ternary (Ru x M z S y ) ruthenium sulphides M being a transition metal preferably selected among W, Co, Mo, Ir, Rh, Cu, Ag and Hg, precipitate in a single crystalline phase with lattice parameters corresponding to a pyrite-type lattice (Pa 3 space group).
• the resulting (Pa 3) RuS2 or Ru x M z S y catalysts turn out to be more active and more stable in the
• M/49021-PCT hydrochloric acid electrolysis conditions than mixed-valence ruthenium sulphide systems of the prior art.
• the preferred catalyst specific loading and selected carbon support are the same applying for rhodium sulphide; also the method of manufacturing is substantially the same, even though suitable temperatures for the thermal treatment may vary from 150 to 1250 0 C.
• the specific reaction pathway of the method according to the invention has the main advantage to intervene on the reduction potentials of the metals and the thionic moieties preventing the formation of discrete reduction states, which are the main factor controlling the yield and proper phase composition of the selected chalcogenide moiety as mentioned above.
• the method of the invention promotes the instantaneous metathesis of the metal ions and the thionic part.
• the disclosed catalysts are suitable for being incorporated in gas-diffusion electrode structures on electrically conductive webs as known in the art.
• Figure 1 shows an X-Ray diffractogram of a rhodium sulphide catalyst prepared according to the method of the invention.
• Described herein is a method to precipitate a rhodium sulphide single crystalline phase on carbon according to the method of the invention; precipitation reactions of other noble metal sulphide catalysts (such as the sulphides of ruthenium, platinum, palladium or iridium) only require minor adjustments that can be easily derived by one skilled in the art.
• other noble metal sulphide catalysts such as the sulphides of ruthenium, platinum, palladium or iridium
• Vulcan XC 72-R high surface area carbon black from Cabot Corporation 7 g were added to the solution, and the mix was sonicated for 1 hour at 40 0 C.
• the rhodium/Vulcan solution was kept at room temperature and stirred vigorously while monitoring the pH.
• the sulphur source and reducing agent solutions were simultaneously added dropwise to the rhodium/Vulcan solution.
• pH, temperature and colour of the solution were monitored. Constant control of the pH is essential in order to avoid the complete dissociation of the thionic compound to elemental S 0 .
• the kinetics of the reaction is very fast, therefore the overall precipitation of the amorphous sulphide occurs within few minutes from the beginning of the reaction. Cooling the reaction can help in slowing the kinetics if needed.
• the reaction was monitored by checking the colour changes: the initial deep pink/orange colour of the
• M/49021-PCT rhodium/Vulcan solution changes dramatically to grey/green (reduction of Rh +3 to Rh +2 species) and then colourless upon completion of the reaction, thus indicating a total absorption of the products on carbon.
• Spot tests were also carried out in this phase at various times with a lead acetate paper; limited amount of H 2 S was observed due to a minimal dissociation of the thionic species.
• the precipitate was allowed to settle and then filtered; the filtrate was washed with 1000 ml deionised water to remove any excess reagent, then a filter cake was collected and air dried at 110 0 C overnight.
• the dried product was finally subjected to heat treatment under flowing argon for 2 hours at 650 0 C, resulting in a weight loss of 22.15%.
• the resulting carbon supported catalyst was first characterised in a corrosion test, to check its stability in a hydrochloric acid electrolysis environment.
• Rh x Sy usually obtained by precipitation is characterised by a balanced phase mixture of at least three Rh-S phases: orthorhombic (Pbcn) Rh 2 Ss, monoclinic (C2/m) Rh 3 S 4 , and primitive cubic (Pm-3m) RhH 7 Si 5 .
• the Rh 2 S 3 phase is an electronic insulator built of alternating RhS ⁇ octahedra.
• the average Rh-Rh bond distance of 3.208 A (compared to 2.69 A in fee Rh metal) thus removes any possibility of direct Rh-Rh bonding.
• the Rhi 7 Si 5 phase possesses semiconductor properties at room temperature.
• RhH 7 Si 5 consists of Rh 8 octahedra with an average Rh-Rh distance of 2.59 A.
• the Rh 3 S 4 phase, with its metallic Rh 6 octahedra eaves, is an active site for O(H) adsorption.
• a ruthenium cobalt ternary sulphide (3:1 ) catalyst was prepared in a similar manner as the one of Example 1 , the difference being that the thionic reagent is now part of the metal ion solution, thus the metathesis reaction occurs in-situ on the metal ion sites.
• the sulphur source solution containing ruthenium, cobalt and Vulcan carbon black was kept at room temperature and stirred vigorously while monitoring the pH. Once the reducing agent solution was prepared, it was added dropwise to the sulphur source solution. During the addition of the reagents, pH, temperature and colour of the solution were monitored. Constant control of the pH is essential in order to avoid the complete dissociation of the thionic compound to elemental S 0 .
• Example 1 also in this case the kinetics of the reaction is very fast therefore the overall precipitation of the amorphous sulphide occurs within few minutes from the beginning of the reaction. Cooling the reaction can help in slowing the kinetics if
• the dried product was finally subjected to heat treatment under flowing nitrogen for 2 hours at 500 0 C, resulting in a weight loss of 32.5%.
• the resulting carbon supported catalyst was subjected to the same corrosion and electrochemical tests of the previous example, showing identical results.
• Example 1 Different samples of the catalysts of Examples 1 and 2 were prepared, mixed to a PTFE dispersion and incorporated into conventional flow-through gas diffusion electrode structures on carbon cloth. All the electrodes were compared to a standard state-of-the-art supported Rh x Sy electrode for hydrochloric acid electrolysis, according to the teaching of US Patent 6,149,782 and 6,967,185 (Sample 0). Such electrodes were tested as oxygen-consuming cathodes in a 50 cm 2 active area laboratory cell against a standard anode, making use of a by-product aqueous hydrochloric acid solution from an isocyanate plant. The overall cell voltage was recorded at two different current densities, namely 3 and 6 kA/m 2 , and the corresponding values are reported in Table 1.
• Equivalent rhodium sulphide catalysts were obtained also by using sodium trithionate, tetrathionate and heptathionate precursors previously prepared according to known procedures, with minor adjustments easily derivable by one skilled in the art. Analogous corrosion and electrochemical results were obtained also in these cases.

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